Australopithecus

Australopithecus (/ˌɒstrələˈpɪθɪkəs/, OS-trə-lə-PITH-i-kəs);[1] from Latin australis, meaning 'southern', and Greek πίθηκος (pithekos), meaning 'ape'; singular: australopith) is a genus of hominins that existed in Africa from around 4.2[2] to 1.9 million years ago and from which the genus Homo, including modern humans, is considered to be descended. Australopithecus is a member of the subtribe Australopithecina,[3][4] which includes Paranthropus, Kenyanthropus,[5] Ardipithecus[5] and Praeanthropus,[6] though the term "australopithecine" is sometimes used to refer only to members of Australopithecus. Species include: A. garhi, A. africanus, A. sediba, A. afarensis, A. anamensis, A. bahrelghazali and A. deyiremeda. Debate exists as to whether other hominid species of this time, such as Paranthropus ('robust australopithecines'), belong to a separate genus or Australopithecus ('gracile australopiths)', or whether some Australopithecus species should be reclassified into new genera.[2]

Australopithecus
Temporal range: Early Pliocene–Early Pleistocene, 4.5–1.9/1.2 mya
↓
Mrs. Ples, an Australopithecus africanus specimen
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Primates
Suborder: Haplorhini
Infraorder: Simiiformes
Family: Hominidae
Subfamily: Homininae
Tribe: Hominini
Subtribe: Australopithecina
Genus: Australopithecus
R.A. Dart, 1925
Type species
†Australopithecus africanus
Dart, 1925
Species

From palaeontological and archaeological evidence, Australopithecus apparently evolved in eastern Africa around 4.2 million years ago before spreading throughout the continent and eventually becoming extinct 1.9 million years ago (or 1.2 million years ago if Paranthropus is included).[7] While none of the groups normally directly assigned to this group survived, Australopithecus does not appear to be literally extinct (in the sense of having no living descendants), as the genus Homo probably emerged from an Australopithecus species[2][8][9][10][11] at some time between 3 and 2 million years ago.[12]

Australopithecus possessed two of three duplicated genes derived from SRGAP2 roughly 3.4 and 2.4 million years ago (SRGAP2B and SRGAP2C), the second of which contributed to the increase in number and migration of neurons in the human brain.[13][14] Significant changes to the hand first appear in the fossil record of later A. afarensis about 3 million years ago (fingers shortened relative to thumb and changes to the joints between the index finger and the trapezium and capitate).[15]

Taxonomy

Research history
Main article: Taung Child
Taung child's skull

The first Australopithecus specimen, the type specimen, was discovered in 1924 in a lime quarry by workers at Taung, South Africa. The specimen was studied by the Australian anatomist Raymond Dart, who was then working at the University of the Witwatersrand in Johannesburg. The fossil skull was from a three-year-old bipedal primate that he named Australopithecus africanus. The first report was published in Nature in February 1925. Dart realised that the fossil contained a number of humanoid features, and so he came to the conclusion that this was an early human ancestor.[16] Later, Scottish paleontologist Robert Broom and Dart set out to search for more early hominin specimens, and several more A. africanus remains from various sites. Initially, anthropologists were largely hostile to the idea that these discoveries were anything but apes, though this changed during the late 1940s.[16] In 1950, evolutionary biologist Ernst Walter Mayr said that all bipedal apes should be classified into the genus Homo, and considered renaming Australopithecus to Homo transvaalensis.[17] However, the contra view taken by Robinson in 1954, excluding australopiths from Homo, became the prevalent view.[17] The first australopithecine fossil discovered in eastern Africa was an A. boisei skull excavated by Mary Leakey in 1959 in Olduvai Gorge, Tanzania. Since then, the Leakey family has continued to excavate the gorge, uncovering further evidence for australopithecines, as well as for Homo habilis and Homo erectus.[16] The scientific community took 20 more years to widely accept Australopithecus as a member of the human family tree.

In 1997, an almost complete Australopithecus skeleton with skull was found in the Sterkfontein caves of Gauteng, South Africa. It is now called "Little Foot" and it is around 3.7 million years old. It was named Australopithecus prometheus[18][19] which has since been placed within A. africanus. Other fossil remains found in the same cave in 2008 were named Australopithecus sediba, which lived 1.9 million years ago. A. africanus probably evolved into A. sediba, which some scientists think may have evolved into H. erectus,[20] though this is heavily disputed.

A. afarensis, A. anamensis, and A. bahrelghazali were split off into the genus Praeanthropus, but this genus has been largely dismissed.[21]

Classification

The genus Australopithecus is considered to be a wastebasket taxon, whose members are united by their similar physiology rather than close relations with each other over other hominin genera. As such, the genus is paraphyletic, not consisting of a common ancestor and all of its descendents, and is considered an ancestor to Homo, Kenyanthropus, and Paranthropus.[22][23][24][25] Resolving this problem would cause major ramifications in the nomenclature of all descendent species. Possibilities suggested have been to rename Homo sapiens to Australopithecus sapiens[26] (or even Pan sapiens[27][28]), or to move some Australopithecus species into new genera.[29]

Opinions differ as to whether the Paranthropus should be included within Australopithecus,[30] and Paranthropus is suggested along with Homo to have developed as part of a clade with A. africanus as its basal root.[17] The members of Paranthropus appear to have a distinct robustness compared to the gracile australopiths, but it is unclear if this indicates all members stemmed from a common ancestor or independently evolved similar traits from occupying a similar niche.[31]

Occasional suggestions have been made (by Cele-Conde et al. 2002 and 2007) that A. africanus should also be moved to Paranthropus.[2] On the basis of craniodental evidence, Strait and Grine (2004) suggest that A. anamensis and A. garhi should be assigned to new genera.[32] It is debated whether or not A. bahrelghazali is simply a western version of A. afarensis and not a separate species.[33][34]

A taxonomy of the Australopithecus within the great apes is assessed as follows, with Paranthropus and Homo emerging among the Australopithecus.[35] The genus Australopithecus with conventional definitions is assessed to be highly paraphyletic, i.e. it is not a natural group, and the genera Kenyanthropus, Paranthropus and Homo are included.[36][37][38]

African hominin timeline (in mya)
View references
HumanHomo nalediHomo rhodesiensisHomo ergasterAustralopithecus sedibaParanthropus robustusParanthropus boiseiHomo rudolfensisHomo habilisAustralopithecus garhiParanthropus aethiopicusLD 350-1Australopithecus bahrelghazaliKenyanthropusAustralopithecus deyiremedaAustralopithecus africanusAustralopithecus afarensisAustralopithecus anamensisArdipithecus ramidusArdipithecus kadabba

Evolution
Map of the fossil sites of the early australopithecines in Africa

A. anamensis may have descended from or was closely related to Ardipithecus ramidus.[39] A. anamensis shows some similarities to both Ar. ramidus and Sahelanthropus.[39]

Australopiths shared several traits with modern apes and humans, and were widespread throughout Eastern and Northern Africa by 3.5 million years ago (mya). The earliest evidence of fundamentally bipedal hominins is a 3.6 Ma fossil trackway in Laetoli, Tanzania, which bears a remarkable similarity to those of modern humans. The footprints have generally been classified as australopith, as they are the only form of prehuman hominins known to have existed in that region at that time.[40]

Australopithecus anamensis, A. afarensis, and A. africanus are among the most famous of the extinct hominins. A. africanus was once considered to be ancestral to the genus Homo (in particular Homo erectus). However, fossils assigned to the genus Homo have been found that are older than A. africanus. Thus, the genus Homo either split off from the genus Australopithecus at an earlier date (the latest common ancestor being either A. afarensis or an even earlier form, possibly Kenyanthropus), or both developed from a yet possibly unknown common ancestor independently.

According to the Chimpanzee Genome Project, the human–chimpanzee last common ancestor existed about five to six million years ago, assuming a constant rate of mutation. However, hominin species dated to earlier than the date could call this into question.[41] Sahelanthropus tchadensis, commonly called "Toumai", is about seven million years old and Orrorin tugenensis lived at least six million years ago. Since little is known of them, they remain controversial among scientists since the molecular clock in humans has determined that humans and chimpanzees had a genetic split at least a million years later. One theory suggests that the human and chimpanzee lineages diverged somewhat at first, then some populations interbred around one million years after diverging.[41]

Anatomy
Reconstruction of a female A. afarensis at CosmoCaixa Barcelona

The brains of most species of Australopithecus were roughly 35% of the size of a modern human brain[42] with an endocranial volume average of 466 cc (28.4 cu in).[12] Although this is more than the average endocranial volume of chimpanzee brains at 360 cc (22 cu in)[12] the earliest australopiths (A. anamensis) appear to have been within the chimpanzee range,[39] whereas some later australopith specimens have a larger endocranial volume than that of some early Homo fossils.[12]

Most species of Australopithecus were diminutive and gracile, usually standing 1.2 to 1.4 m (3 ft 11 in to 4 ft 7 in) tall. It is possible that they exhibited a considerable degree of sexual dimorphism, males being larger than females.[43] In modern populations, males are on average a mere 15% larger than females, while in Australopithecus, males could be up to 50% larger than females by some estimates. However, the degree of sexual dimorphism is debated due to the fragmentary nature of australopith remains.[43]

According to A. Zihlman, Australopithecus body proportions closely resemble those of bonobos (Pan paniscus),[44] leading evolutionary biologist Jeremy Griffith to suggest that bonobos may be phenotypically similar to Australopithecus.[45] Furthermore, thermoregulatory models suggest that australopiths were fully hair covered, more like chimpanzees and bonobos, and unlike humans.[46]

The fossil record seems to indicate that Australopithecus is ancestral to Homo and modern humans. It was once assumed that large brain size had been a precursor to bipedalism, but the discovery of Australopithecus with a small brain but developed bipedality upset this theory. Nonetheless, it remains a matter of controversy as to how bipedalism first emerged. The advantages of bipedalism were that it left the hands free to grasp objects (e.g., carry food and young), and allowed the eyes to look over tall grasses for possible food sources or predators, but it is also argued that these advantages were not significant enough to cause the emergence of bipedalism. Earlier fossils, such as Orrorin tugenensis, indicate bipedalism around six million years ago, around the time of the split between humans and chimpanzees indicated by genetic studies. This suggests that erect, straight-legged walking originated as an adaptation to tree-dwelling.[47] Major changes to the pelvis and feet had already taken place before Australopithecus.[48] It was once thought that humans descended from a knuckle-walking ancestor,[49] but this is not well-supported.[50]

Australopithecines have thirty two teeth, like modern humans. Their molars were parallel, like those of great apes, and they had a slight pre-canine gap (diastema). Their canines were smaller, like modern humans, and with the teeth less interlocked than in previous hominins. In fact, in some australopithecines, the canines are shaped more like incisors.[51] The molars of Australopithicus fit together in much the same way those of humans do, with low crowns and four low, rounded cusps used for crushing. They have cutting edges on the crests.[51] However, australopiths generally evolved a larger postcanine dentition with thicker enamel.[52] Australopiths in general had thick enamel, like Homo, while other great apes have markedly thinner enamel.[51] Robust australopiths wore their molar surfaces down flat, unlike the more gracile species, who kept their crests.[51]

Diet

In a 1979 preliminary microwear study of Australopithecus fossil teeth, anthropologist Alan Walker theorized that robust australopiths ate predominantly fruit (frugivory).[53] Australopithecus species are thought to have eaten mainly fruit, vegetables, and tubers, and perhaps easy to catch animals such as small lizards. Much research has focused on a comparison between the South African species A. africanus and Paranthropus robustus. Early analyses of dental microwear in these two species showed, compared to P. robustus, A. africanus had fewer microwear features and more scratches as opposed to pits on its molar wear facets.[54] Microwear patterns on the cheek teeth of A. afarensis and A. anamensis indicate that A. afarensis predominantly ate fruits and leaves, whereas A. anamensis included grasses and seeds (in addition to fruits and leaves).[55] The thickening of enamel in australopiths may have been a response to eating more ground-bound foods such as tubers, nuts, and cereal grains with gritty dirt and other small particulates which would wear away enamel. Gracile australopiths had larger incisors, which indicates tearing food was important, perhaps eating scavenged meat. Nonetheless, the wearing patterns on the teeth support a largely herbivorous diet.[51]

In 1992, trace-element studies of the strontium/calcium ratios in robust australopith fossils suggested the possibility of animal consumption, as they did in 1994 using stable carbon isotopic analysis.[56] In 2005, fossil animal bones with butchery marks dating to 2.6 million years old were found at the site of Gona, Ethiopia. This implies meat consumption by at least one of three species of hominins occurring around that time: A. africanus, A. garhi, and/or P. aethiopicus.[57] In 2010, fossils of butchered animal bones dated 3.4 million years old were found in Ethiopia, close to regions where australopith fossils were found.[58]

Robust australopithecines (Paranthropus) had larger cheek teeth than gracile australopiths, possibly because robust australopithecines had more tough, fibrous plant material in their diets, whereas gracile australopiths ate more hard and brittle foods.[51] However, such divergence in chewing adaptations may instead have been a response to fallback food availability. In leaner times, robust and gracile australopithecines may have turned to different low-quality foods (fibrous plants for the former, and hard food for the latter), but in more bountiful times, they had more variable and overlapping diets.[59][60]

A study in 2018 found non-carious cervical lesions, caused by acid erosion, on the teeth of A. africanus, probably caused by consumption of acidic fruit.[61]

Technology

It was once thought that Australopithecus could not produce tools like Homo, but the discovery of A. garhi associated with large mammal bones bearing evidence of processing by stone tools showed this to not have been the case.[62][63] Discovered in 1994, this was the oldest evidence of manufacturing at the time[64][65] until the 2010 discovery of cut marks dating to 3.4 mya attributed to A. afarensis,[66] and the 2015 discovery of the Lomekwi culture from Lake Turkana dating to 3.3 mya possibly attributed to Kenyanthropus.[67] More stone tools dating to about 2.6 mya in Ledi-Geraru in the Afar Region were found in 2019, though these may be attributed to Homo.[68]

Notable specimens
  • KT-12/H1, an A. bahrelghazali mandibular fragment, discovered 1995 in Sahara, Chad
  • AL 129-1, an A. afarensis knee joint, discovered 1973 in Hadar, Ethiopia
  • Karabo, a juvenile male A. sediba, discovered in South Africa
  • Laetoli footprints, preserved hominin footprints in Tanzania
  • Lucy, a 40%-complete skeleton of a female A. afarensis, discovered 1974 in Hadar, Ethiopia
  • Selam, remains of a three-year-old A. afarensis female, discovered in Dikika, Ethiopia
  • STS 5 (Mrs. Ples), the most complete skull of an A. africanus ever found in South Africa
  • STS 14, remains of an A. africanus, discovered 1947 in Sterkfontein, South Africa
  • STS 71, skull of an A. africanus, discovered 1947 in Sterkfontein, South Africa
  • Taung Child, skull of a young A. africanus, discovered 1924 in Taung, South Africa
  • The spot where the first Australopithecus boisei was discovered in Tanzania.
  • Original skull of Mrs. Ples, a female A. africanus
  • Taung Child by Cicero Moraes, Arc-Team, Antrocom NPO, Museum of the University of Padua.
  • Cast of the skeleton of Lucy, an A. afarensis
  • Skull of the Taung child
gollark: Definitely. Not that those are contradictory.
gollark: It's probably best to just write it without much time spent trying to eke out maximum performance.
gollark: My site runs on an old tower server and is at 1% CPU almost all the time. The biggest CPU load is just the monitoring software gathering a few hundred metrics for graphs.
gollark: That would take much more work, and individual computers are very fast.
gollark: Generally, no.

See also

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